1. Field of the Invention
The present invention relates to a method of manufacturing a center electrode for a spark plug adapted to be assembled in an internal combustion engine.
2. Description of the Related Art
Conventionally, a center electrode for spark plugs includes a metal cup formed into a bottomed hollow cylinder and a core member inserted into the metal cup. The core member is made of metal having a higher thermal conductivity than the cup. The center electrode has a fore end formed with a small-diameter portion. The small-diameter portion is formed by a cutting or turning process.
Formation of the small-diameter portion by cutting operation, however, requires a relatively long machining time and hence the machining cost increases correspondingly. One prior approach taken to obviate the need for cutting operation is known as disclosed in, for example, Japanese Patent Laid-open Publication (JP-A) No. 09-120882. According to the disclosed approach, a metal cup is forged to form a small-diameter portion and, thereafter, a core member is press-fitted in the metal cup. A problem is that the small-diameter portion forms a bearing surface which receives a press-fitting load or pressure during press-fitting operation. The small-diameter portion is, therefore, likely to deform. An attempt to lower the press-fitting load to thereby suppress deformation of the small-diameter portion has been made, but the result is not fully satisfactory in that due to insufficient adhesion between a bottom portion of the metal cup and a fore end portion of the core member, thermal conductivity of the center electrode is deteriorated.
With the foregoing difficulties in view, an object of the present invention is to provide a method which is capable of manufacturing a center electrode at a relatively low machining cost, with excellent accuracy in shape of a small-diameter portion, and with good adhesion between a metal cup and a core member,